Fatigue Analysis of Offshore Drilling Unit

Transcription

1 Fatigue Analysis of Offshore Drilling Unit Master Thesis presented in partial fulfillment of the requirements for the double degree: Advanced Master in Naval Architecture conferred by University of Liege "Master of Sciences in Applied Mechanics, specialization in Hydrodynamics, Energetics and Propulsion conferred by Ecole Centrale de Nantes developed at West Pomeranian University of Technology, Szczecin in the framework of the EMSHIP Erasmus Mundus Master Course in Integrated Advanced Ship Design Ref BE-ERA MUNDUS-EMMC Supervisor: Reviewer: Prof. Maciej Taczala, West Pomeranian University of Technology, Szczecin Prof. Hervé Le Sourne, ICAM Szczecin, February

2 P 2 Contents DECLARATION OF AUTHORSHIP... 6 ABBREVIATIONS... 7 ABSTRACT... 8 SHORT DESCRIPTION INTRODUCTION Background Objective Methodology Schedule Types of Fatigue Failure: Sources of Fatigue: OFFSHORE DRILLING PLATFORMS: Introduction: Components of Offshore Rigs: Category: STUCTURE ANALYZED: SEMISUMMERSIBLE UNIT Introduction: Classification: Example of similar model: SOFTWARE PROCEDURE: Introduction: Sesam Genie: HydroD-Wadam: Sestra: Xtact: Postresp: Stofat: STRUCTURAL MODELLING Modelling Set-Up Pontoon: Column: Deck: Derrick: Boundary Conditions: Panel Model: Master Thesis developed at West Pomeranian University of Technology, Szczecin

3 Fatigue Analysis of Offshore Drilling Unit Morison Model: Structural Model: HYDRODYNAMIC ANALYSIS: Analysis Setup Global Motion Response Analysis: GLOBAL STRUCTURAL STRENGTH ANALYSIS GLOBAL FATIGUE ANALYSIS AND RESULT: CONCLUSIONS AND RECOMMENDATIONS: REFERENCES APPENDIX Appendix A: Summary of Model Properties Appendix B: Element Fatigue Check Result ACKNOWLEDGEMENTS EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

4 P 4 List Of Figures Figure 1 Flowchart of Methodology Figure 2 Detail Schedule with Gantt chart Figure 3 Component of offshore rigs (19) Figure 4 Common Types of Drilling Rigs (BOEMRE, 2010c) Figure 5 Fixed Platform Rig (Left) and Gravity-Based Structure (Right) (Source: Prof: Tadeusz Graczyk Lectures at ZUT) Figure 6 Varieties of mobile offshore drilling units (MODUs). Drill Barge (TODCO via NETL, 2011), Jack-Up Rig (Transocean, 2011), Semi-submersible Rig (Eni, 2008), Drill Ship (BP p.l.c., 2011) Figure 7 Tension Leg Platform (Magnolia TLP, Source: Prof: Tadeusz Graczyk Lectures at ZUT) Figure 8 Column Stabilized Semisubmersibles (Left: Ring Pontoon Design, Right: Twin Pontoon Design, Source: Petrowiki) Figure 9 Similar Drilling Platform model (Maersk Drilling deep-water semi-submersible rig Source: Prof: Tadeusz Graczyk Lectures at ZUT) Figure 10 Semi-submersible platform - Maersk Drilling deep-water semi-submersible rig (Source: Prof: Tadeusz Graczyk Lectures at ZUT) Figure 11 Flowchart of Software Procedure Figure 12 Schematic illustration of the capabilities of Sestra (12) Figure 13 Color coding of structural members Figure 14 Color coding of thickness Figure 15 Pontoon Figure 16 Column Figure 17 Deck Figure 18 Deck Framing System Figure 19 Derrick Figure 20 Boundary conditions (Source: DNV-RP-C103_ ) Figure 21 Boundary conditions applied on pontoons Figure 22 Panel Model Figure 23 Wet Surface for Hydrodynamic Analysis Figure 24 Meshed Panel Model Figure 25 Morison Meshed Model Figure 26 Structural Mesh Model Figure 27 Problematic mesh elements Figure 28 Final Structural Mesh Model Figure 29 Hydro Model of Semisubmersible Drilling Platform Figure 30 Four compartments inside two pontoons Figure 31 Off body points to define sea state grid Figure 32 Mass model of the Drilling Unit Figure 33 RAO for different wave directions at relative points (0, 0, and 35) Figure 34 RAO of Heave Figure 35 RAO of Roll Figure 36 RAO of Pitch Figure 37 Damping Matrix Figure 38 RAO at relative point (0, 0, and 35) after addition of damping Figure 39 Surface wave loads at 180 degree heading and Hz frequency (Pitch RAO). 51 Figure 40 Surface wave loads at 270 degree heading and Hz frequency (Heave RAO) 51 Figure 41Surface wave loads at 270 degree heading and Hz frequency (Roll RAO) Figure 42 Surface wave loads at 180 degree heading and Hz frequency (Surge RAO) 52 Figure 43 Surface wave loads at 270 degree heading and Hz frequency (Sway RAO). 53 Master Thesis developed at West Pomeranian University of Technology, Szczecin

5 Fatigue Analysis of Offshore Drilling Unit 5 Figure 44 Surface wave loads at 315 degree heading and Hz frequency (YAW RAO) 53 Figure 45 Global structural model Figure 46 Von-Misses Stress at 45 degree wave heading and Hz excitation frequency Figure 47 Von-Misses Stress on Column Figure 48 Scatter Diagram for the North Atlantic (Source: DNV-RP-C205) Figure 49 DNV-SN Curves (Stofat_UM) Figure 50 DNVC-I SN curve plotted from STOFAT Figure 51 Wave Spreading function for short crested Sea Figure 52 Definition of the wave direction (heading angle) in this investigation Figure 53 Maximum Usage Factor of the Structure Figure 54 Fatigue Life of Global Structure List of Tables Table 1 Main Dimensional Parameter for Semisubmersible Analyzed Table 2 Material Properties of the Structural Model (St52) Table 3 Sea State Direction Set Table 4 Mass Model Properties of the Structure Table 5 Load cases for Heading and Wave periods Table 6 Long term response from Postresp Table 7 Fatigue life in Critical connections EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

6 P 6 DECLARATION OF AUTHORSHIP I declare that this thesis and the work presented in it are my own and have been generated by me as the result of my own original research. Where I have consulted the published work of others, this is always clearly attributed. Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. I have acknowledged all main sources of help. Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself. This thesis contains no material that has been submitted previously, in whole or in part, for the award of any other academic degree or diploma. I cede copyright of the thesis in favour of the West Pomeranian University of Technology Szczecin, Poland. Date: Signature Master Thesis developed at West Pomeranian University of Technology, Szczecin

7 Fatigue Analysis of Offshore Drilling Unit 7 ABBREVIATIONS CN COG CT DNV GL Hs FE FP FEA JP L-File MODU NA OS RAO RP Tp TLP UM Classification Notes Centre of Gravity Compliant Tower Platforms Det Norske Veritas Germanischer Lloyd Significant Height Finite Element Fixed Platform Finite Element Analysis Jack-up Platform Load Interface File Mobile offshore drilling units North Atlantic Offshore Standard Response Amplitude Operator Recommended Practice Peak Period Tension Leg Platform User Manual EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

8 P 8 ABSTRACT Drilling operation in deep water, harsh environment and remote locations becomes a key trend for the offshore industry to fulfil increasing demand for energy. For operational conditions wave induced loads are more significant for the offshore installations. Therefore, to ensure integrity and structural safety, the wave induced loads have to be taken into account. One of the approaches to accomplish this task is to perform a fatigue analysis with the extreme environmental loading on the offshore platform using rules and practices recommended by classification societies. The main objective of the thesis has been to present a case study of a semisubmersible drilling unit regarding the fatigue analysis. The applied approach consisted in finite element modelling of the global structure, applying hydrodynamic loads using recommended offshore design codes, transferring wave loads from hydrodynamic model to structural model and perform the fatigue analysis with most unfavorable combination of environmental conditions. Among different methods of fatigue analysis, the spectral method is considered as most suitable in which long term distribution of stresses is calculated using wave scatter data. For finite element modelling SESAM Genie was used while HydroD Wadam was used to analyze hydrodynamic loads and also transfer the loads to the finite element model for subsequent structural analysis. These hydrodynamic loads were applied for a number of wave directions and for a range of wave frequencies covering the necessary sea states and the results in form of stresses were obtained. These results were then used to calculate fatigue damages at given points in the structural model using another software Stofat. Master Thesis developed at West Pomeranian University of Technology, Szczecin

9 Fatigue Analysis of Offshore Drilling Unit 9 SHORT DESCRIPTION A short description of this whole paper is given below: This paper contains nine chapters in total. First chapter described the introduction of the project including what is to be achieved, why this project needs to be done, methodology of the project and detail time allocation with chart. Second chapter contains a brief overview of the offshore drilling structures, different types of offshore structures, their basis of applications and operation In third chapter, the description of Semisubmersible platform which is used as a case study for the current master thesis has been given with brief overview, types and example. Fourth chapter includes a brief description of software tool used for this thesis and flowchart of methodology. Fifth chapter presents the global structural modeling of the drilling unit studied in detail. It describes the 3d modelling for FE analysis with material properties, meshing and boundary conditions to perform subsequent analysis Sixth chapter contains Hydrodynamic Analysis of the structure including detailed overview of analysis setup, mass model properties and subsequent global motion response for the given set of extreme environmental loading conditions. Global structural strength is presented on Seventh chapter with detailed load case and vonmisses stress Finally global fatigue analysis result is listed in eighth chapter with SN-curve and scatter data of the desired location. Ninth is a conclusion chapter, which deals with the initial aim and objectives, achievement and some suggestions for the future development of the work. References and some important appendices are attached at the end. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

10 P INTRODUCTION 1.1 Background As drilling extended further offshore into deeper water to access additional energy resources, the structures are also largely exposed to stresses which induced by time variation. These type of stress pattern are the forces generated principally by the sea waves. These loads are repeated for thousands of cycles through the lifetime of the structure. After many cycles the accumulated damage reduces the ability of the structural member to withstand loading. Global Fatigue analysis is one of the approach to quantify wave induced load effects to ensure integrity and structural safety of the offshore platform. A methodology has been developed for global fatigue analysis of an offshore drilling unit with extreme environmental loading condition. 1.2 Objective The main objective is to determine screening fatigue lives based upon DNV S-N curve and wave scatter data. The calculated fatigue life indicate the distribution of fatigue sensitive area. In order to meet the objective, the following sub-targets are to be fulfilled for the Global Model analysis: Make a 3D-model of the structure for FE analysis Calculate the hydrodynamic loads in the frequency domain. Identify critical locations with respect to von Mises stress for different wave direction. Identify fatigue critical locations using linear FE-analysis. Perform the analysis for different wave directions (heading angles). 1.3 Methodology The objective with this study is to simulate numerically and analyze the structure response followed by the fatigue life of an offshore drilling unit considering wave direction, magnitude of wave loads and the location of interest. Following numerical analysis have been performed: Hydrodynamic Analysis Structural Response Analysis Fatigue Analysis Figure 1 depicts the methodology of the thesis and all the steps has been described briefly on later sections. Master Thesis developed at West Pomeranian University of Technology, Szczecin

11 Fatigue Analysis of Offshore Drilling Unit 11 3D-Modelling (Sesam-GeniE) Hydrodynamic Analysis (HydroD-Wadam) Structural Analysis (Sestra) Global Response (Xtract) Fatigue Analysis (Stofat) Figure 1 Flowchart of Methodology For the FE modeling and analysis of the drilling unit, DNV software Sesam GeniE has been used. The hydrodynamic simulation is carried out in the DNV software HydroD Wadam. The hydrodynamic simulations are performed in the frequency domain for the structure operating in north Atlantic. For the frequency domain, a Bretschneider spectrum with 26 frequencies and 8 wave directions are chosen. The global motion response of the structure was analyzed using a post-processing software named POSTRESP. The FE-simulations are carried out in the DNV software SESTRA for linear structural FE-analysis. The fatigue analysis was done in STOFAT and results are graphically presented with XTRACT. A brief description of software used for analysis is given in Chapter four. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

12 P Schedule Detail schedule of the thesis is presented below with Microsoft Project Gantt chart. Figure 2 Detail Schedule with Gantt chart 1.5 Types of Fatigue Failure: Two categories of fatigue damage are generally recognized and they are termed high frequency and low frequency fatigue. In high frequency fatigue, failure is initiated in the form of small cracks, which grow slowly and which may often be detected and repaired before the structure is endangered. High frequency fatigue involves several millions of cycles of relatively low stress (less than yield) and is typically encountered in machine parts rotating at high speed or in structural components exposed to severe and prolonged vibration. Low frequency fatigue involves higher stress levels, up to and beyond yield, which may result in cracks being initiated after several thousand cycles. Master Thesis developed at West Pomeranian University of Technology, Szczecin

13 Fatigue Analysis of Offshore Drilling Unit Sources of Fatigue: Cyclic Load Whipping Springing Engines and propeller Cyclic Loads: Offshore structures of all types are generally subjected to cyclic loading from wind, current and waves. Dynamic wave produce stress fluctuations in the structural members and joints and are the primary cause of fatigue damages. In deep water environments wind loads represent a contribution of about 5 % to the environmental loading. Current loads are mostly considered to be unimportant in the dynamic analysis of offshore structures, because their frequencies are not sufficient to excite the structures. Wave loads are considered as the main source of excitation for current piece of work Whipping: Shocks between wave and ship bow is known as slamming, this shocks generates vibration which is known as Whipping. So it is induced by wave impacts under the ship s flared bow, the overhanging stern, or the bottom, leads to transient, decaying hull girder vibrations which typically occur in moderate or harsh seaways. For the current thesis work whipping is not taken into account because of deep-water consideration and non-flared bow Springing: Springing is caused by regular, periodic wave trains that excite resonant hull girder vibrations occurring in low to moderate seaways. If excitation of waves equal to 1st beam or 2nd beam natural frequency of structure then resonant occur and passenger moves/ jumps with structure. As springing is negligible in deep-water, it is not considered for current work Engine Excitation Frequency: High frequency response coming from engine or propeller creates severe vibration and initiate fatigue cracks on nearby parts. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

14 P OFFSHORE DRILLING PLATFORMS: 2.1 Introduction: One of the remarkable accomplishments of the petroleum industry has been the development of technology that allows for drilling wells offshore to access additional energy resources. The basic offshore wellbore construction process is not significantly different than the rotary drilling process used for land based drilling. The main differences are the type drilling rig and modified methods used to carry out the operations in a more complex situation. Depending on the circumstances, the platform may be fixed to the ocean floor, may consist of an artificial island, or may float. For offshore drilling a mechanically stable offshore platform or floating vessel from which to drill must be provided. These range from permanent offshore fixed or floating platforms to temporary bottom-supported or floating drilling vessels. Despite an increase in complexity, improvements in drilling technology have allowed more complex well patterns to be drilled to a greater depth such that additional hydrocarbon resources can be developed at a greater distance from the drilling or production structure, allowing more energy to be produced with less environmental impact. 2.2 Components of Offshore Rigs: Following are the major components of the offshore rigs as presented on figure 3 as well. Hull initially rigs were built out of tanker hulls, so the terminology remains same Power Module converts available fuel into power for the station Process Module onboarding and offloading of supplies and products Drilling Module the traditional drilling rig apparatus Quarters Module where the crew sleeps and eats Wellbay Module access to the well and other equipment Derrick the oil derrick Master Thesis developed at West Pomeranian University of Technology, Szczecin

15 Fatigue Analysis of Offshore Drilling Unit 15 Figure 3 Component of offshore rigs (19) 2.3 Category: There are two basic categories of offshore drilling rigs those that can be moved from place to place, allowing for drilling in multiple locations, and those rigs that are temporarily or permanently placed on a fixed-location platform (platform rigs). Jack-ups, semisubmersibles and drill-ships make up the majority of the offshore rig fleet and all are used worldwide. Other rig types such as platform rigs, inland barges and tender-assisted rigs are used as well, but they are fewer in number and are generally used in specific geographic areas. Common types of offshore platforms are listed below: Jack-up drilling rig Fixed platform Gravity-based structure Compliant Tower Tension-leg platform Spar platform Semi-submersible platform Drillship EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

16 P 16 Figure 4 Common Types of Drilling Rigs (BOEMRE, 2010c) Common types of drilling rigs are presented with figure 4 with a brief description below: Fixed platforms are built on concrete or steel legs, or both, anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are, by virtue of their immobility, designed for very long term use (for instance the Hibernia platform). Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed. Concrete caisson structures, pioneered by the Condeep concept, often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore and then floated to their final position where they are sunk to the seabed. Fixed platforms are economically feasible for installation in water depths up to about 1,700 ft (520 m) Master Thesis developed at West Pomeranian University of Technology, Szczecin

17 Fatigue Analysis of Offshore Drilling Unit 17 Figure 5 Fixed Platform Rig (Left) and Gravity-Based Structure (Right) (Source: Prof: Tadeusz Graczyk Lectures at ZUT) A Gravity Based Structure can either be steel or concrete and is usually anchored directly onto the seabed. Steel GBS are predominantly used when there is no or limited availability of crane barges to install a conventional fixed offshore platform, for example in the Caspian Sea. There are several steel GBS in the world today (e.g. offshore Turkmenistan Waters (Caspian Sea) and offshore New Zealand). Steel GBS do not usually provide hydrocarbon storage capability. These structures are generally feasible in shallow water depth till 100 m although the deepest GBS being used at Troll field in Norway at water depth of 303 m. A compliant tower (CT) is a fixed rig structure normally used for the offshore production of oil or gas. The rig consists of narrow, flexible (compliant) towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 1,500 and 3,000 feet (450 and 900 m). EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

18 P 18 At present the deepest is Petronius in 535 m of water (the tallest freestanding structure in the world.), in operation since With the use of flex elements such as flex legs or axial tubes, resonance is reduced and wave forces are de-amplified. This type of rig structure can be configured to adapt to existing fabrication and installation equipment. Compared with floating systems, such as Tension-leg platforms and SPARs, the production risers are conventional and are subjected to less structural demands and flexing. This flexibility allows it to operate in much deeper water, as it can 'absorb' much of the pressure exerted on it by the wind and sea. It can deflect (sway) in excess of 2% of height. Despite its flexibility, the compliant tower system is strong enough to withstand hurricane conditions. Mobile Offshore Drilling Unit (MODU) are drilling rigs that are used exclusively to drill offshore and that float either while drilling or when being moved from location to another. They fall into two general types: bottom-supported and floating drilling rigs. Bottom-supported drilling rigs are barges or jack-ups. Floating drill rigs include submersible and semi-submersible units and drill ships. Various MODUs are presented on Figure 6. A drilling barge consists of a barge with a complete drilling rig and ancillary equipment constructed on it. Drilling barges are suitable for calm shallow waters (mostly inland applications) and are not able to withstand the water movement experienced in deeper, open water situations. When a drilling barge is moved from one location to another, the barge floats on the water and is pulled by tugs. When a drilling barge is stationed on the drill site, the barge can be anchored in the floating mode or in some way supported on the bottom. The bottom-support barges may be submerged to rest on the bottom or they may be raised on posts or jacked-up on legs above the water. The most common drilling barges are inland water barge drilling rigs that are used to drill wells in lakes, rivers, canals, swamps, marshes, shallow inland bays, and areas where the water covering the drill site in not too deep. Master Thesis developed at West Pomeranian University of Technology, Szczecin

19 Fatigue Analysis of Offshore Drilling Unit 19 Figure 6 Varieties of mobile offshore drilling units (MODUs). Drill Barge (TODCO via NETL, 2011), Jack-Up Rig (Transocean, 2011), Semi-submersible Rig (Eni, 2008), Drill Ship (BP p.l.c., 2011). Submersible drilling rigs are similar to barge rigs but suitable for open ocean waters of relative shallow depth. The drilling structure is supported by large submerged pontoons that are flooded and rest on the seafloor when drilling. After the well is completed, the water is pumped out of the tanks to restore buoyancy and the vessel is towed to the next location. Jack-up drilling rigs are similar to a drilling barge because the complete drilling rig is built on a floating hull that must be moved between locations with tug boats. Jack-ups are the most common offshore bottom-supported type of drilling rig. There are two jackup types; independent-leg jack-ups make up the majority of the existing fleet. They have legs that penetrate into the seafloor and the hull jacks up and down the legs. Matsupported jack-ups are as the name implies, the mat rests on the seafloor during drilling EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

20 P 20 operations. Cantilever jack-ups are able to skid out over the platform or well location, while slot units have a slot that fits around a platform when drilling development wells. Once on location, a jack-up rig is raised above the water on legs that extend to the seafloor for support. Jack-ups can operate in open water or can be designed to move over and drill though conductor pipes in a production platform. These MODU's-Mobile Offshore Drilling Units are typically used in water depths up to 400 feet (120 m), although some designs can go to 550 ft (170 m) depth. They are designed to move from place to place, and then anchor themselves by deploying the legs to the ocean bottom using a rack and pinion gear system on each leg. Semisubmersibles are a common type of floating structure used in the exploration and production of offshore hydrocarbons. These platforms have hulls of sufficient buoyancy to cause the structure to float, but the structural/equipment weight of the platform and the mooring system keeps the structure upright. Typically, four to eight vertical, surface piercing columns are connected to these pontoons. The columns themselves may have cross and horizontal bracing to provide structural strength and triangulated rigidity for the platform. The minimal water plane area contributed by the vertical columns results in long heave, pitch and roll natural periods and the hydrodynamic loading can be minimized at the dominant wave period by careful selection of pontoon volume and water plane area. A more detailed description of this type of offshore platforms has been discussed in the Chapter 3 of this thesis. A drillship is a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory offshore drilling of new oil or gas wells in deep water or for scientific drilling. The drillship can also be used as a platform to carry out well maintenance or completion work such as casing and tubing installation or subsea tree installations. It is often built to the design specification of the oil Production Company and/or investors, but can also be a modified tanker hull outfitted with a dynamic positioning system to maintain its position over the well. The greatest advantages these modern drill ships have is their ability to drill in water depths of more than 2500 meters and the time saved sailing between oilfields worldwide. Drill ships are completely independent, in contrast to semi-submersibles and jack up barges. In order to drill, a Master Thesis developed at West Pomeranian University of Technology, Szczecin

21 Fatigue Analysis of Offshore Drilling Unit 21 marine riser is lowered from the drillship to the seabed with a blowout preventer (BOP) at the bottom that connects to the wellhead. TLPs are floating platforms tethered to the seabed in a manner that eliminates most vertical movement of the structure. TLPs are used in water depths up to about 6,000 feet (2,000 m). The "conventional" TLP is a 4-column design which looks similar to a semisubmersible. Proprietary versions include the Seastar and MOSES mini TLPs; they are relatively low cost, used in water depths between 600 and 4,300 feet (180 and 1,300 m). Mini TLPs can also be used as utility, satellite or early production platforms for larger deep-water discoveries. An example of TLP is given below. Figure 7 Tension Leg Platform (Magnolia TLP, Source: Prof: Tadeusz Graczyk Lectures at ZUT) Spars are moored to the seabed like TLPs, but whereas a TLP has vertical tension tethers, a spar has more conventional mooring lines. The spar has more inherent stability than a TLP since it has a large counterweight at the bottom and does not depend on the mooring to hold it upright. It also has the ability, by adjusting the mooring line tensions (using chain-jacks attached to the mooring lines), to move horizontally and to position itself over wells at some distance from the main platform location. The first production spar was Kerr-McGee's Neptune, anchored in 1,930 ft (590 m) in the Gulf of Mexico; however, spars (such as Brent Spar) were previously used as FSOs. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

22 P STUCTURE ANALYZED: SEMISUMMERSIBLE UNIT 3.1 Introduction: For current work Semi-submersible drilling unit is used which are the most common type of offshore floating drilling rigs and can operate in deep water and usually move from location to location under their own power. These platforms have hulls (columns and pontoons) of sufficient buoyancy to cause the structure to float, but of weight sufficient to keep the structure upright; can be ballasted up or down by altering the amount of flooding in buoyancy tanks. Semis as they are called as the have columns that are ballasted to remain on location either by mooring lines attached to seafloor anchors or may be held in place by adjustable thrusters which are rotated to hold the vessel over the desired location known as dynamically positioned. Semi-submersibles can be used in water depths from 200 to 10,000 feet (60 to 3,000 m). 3.2 Classification: Most common design of semisubmersible rigs are the column-stabilized semisubmersible unit where two horizontal pontoons are connected via cylindrical or rectangular columns to the drilling deck above the water. Column stabilized semisubmersible units design can be classified as follows (figure 8) Ring Pontoon Semisubmersibles: Ring pontoon designs normally have one continuous lower hull (pontoons and nodes) supporting 4-8 vertical columns. The vertical columns are supporting the upper deck. Twin Pontoon Semisubmersibles: Twin pontoon designs normally have two lower pontoons, each supporting 2-4 vertical columns. The 4-8 vertical columns are supporting the upper deck. In addition it may be strengthened with diagonal braces supporting the deck and horizontal braces connecting the pontoons or columns. Master Thesis developed at West Pomeranian University of Technology, Szczecin

23 Fatigue Analysis of Offshore Drilling Unit 23 Figure 8 Column Stabilized Semisubmersibles (Left: Ring Pontoon Design, Right: Twin Pontoon Design, Source: Petrowiki) 3.3 Example of similar model: For the current thesis, simplified model of a twin pontoon column stabilized semisubmersible unit is considered which consist of 4 sets of columns legs, 2 horizontal pontoons, 2 bracings and a drilling derrick that has been chosen to perform fatigue analysis. In Figure below a similar existing model of the semisubmersible drilling unit has been shown Figure 9 Similar Drilling Platform model (Maersk Drilling deep-water semi-submersible rig Source: Prof: Tadeusz Graczyk Lectures at ZUT) EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

24 P 24 Figure 9 and 10 represent Column stabilized dynamically positioned semisubmersible drilling rigs with capability to attach to an 8-point pre-installed mooring system; provisions to attach to a 12- point pre-installed mooring system. Figure 10 Semi-submersible platform - Maersk Drilling deep-water semi-submersible rig (Source: Prof: Tadeusz Graczyk Lectures at ZUT) Main technical dimensions of analyzed structure of current thesis are listed below in Table 1 Table 1 Main Dimensional Parameter for Semisubmersible Analyzed Parameters Characteristic Length= Length of Pontoon Height of Pontoon Width of Pontoon Height of Column Diameter of Column Height of Deck Spacing of Columns, center to center Technical Data 80.6 m 7.5 m 16 m 33.5 m 12.9 m 8 m m Master Thesis developed at West Pomeranian University of Technology, Szczecin

25 Fatigue Analysis of Offshore Drilling Unit SOFTWARE PROCEDURE: 4.1 Introduction: SESAM software package developed by DNV is used for modelling and analysis of the structure. The DNV software package SESAM consists of different modules which depend on the simulation that is supposed to be carried out. The following SESAM-software is used; GeniE, HydroD-WADAM, SESTRA, POSTRESP, STOFAT and XTRACT. Flowchart and brief description of software procedure is given below: Global MOdel-GeniE (T1.FEM) Wadam (G1.SIF, L1.FEM) Postresp Sestra (R1.SIN) Xtract Stofat.Vtf Xtract Figure 11 Flowchart of Software Procedure 4.2 Sesam Genie: The Sesam GeniE software is a software tool for designing and analyzing offshore and maritime structures made of beams and plates. Modelling, analysis and results processing are performed in the same graphic user interface. The use of concept technology makes the Sesam GeniE software highly efficient for integrating stability, loading, strength assessment and CAD exchange. All data are persistent enabling the engineers to do efficient iterative re-design of a structure. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

26 P 26 For floating structures, the Sesam GeniE software can perform static and dynamic linear analysis for structures subjected to wave, wind, current, and ballast and equipment layout. The loads and accelerations from the waves and compartment content are defined by the Sesam HydroD software and they are automatically applied to the structure model independent of the hydrodynamic panel model. The wave loads create input to fatigue assessments of both beams and plates using a stochastic approach. By using the sub-modelling techniques it is very easy to perform a global fatigue analysis to scan for critical areas GeniE may be used as a stand-alone tool using a direct analysis approach which also include: Finite element mesh generation Finite element analysis Finite element results visualization Environmental loads calculation Code checking and rule based design Openness towards leading CAD vendors 4.3 HydroD-Wadam: The Sesam HydroD software is a tool for hydrostatic and hydrodynamic analysis. For the hydrodynamic part of analysis a sub module included in the HydroD package named as Wadam has been used. Wadam is a general analysis program for calculation of wave-structure interaction for fixed and floating structures of arbitrary shape, structures and ship hulls. The Wadam software is based on widely accepted linear methods for marine hydrodynamics, the 3-D radiation-diffraction theory employing a panel model and Morison equation in linearized form employing a beam model. These analyses are normally performed in the frequency domain, but it is also possible to do it in time domain (Linear as well as nonlinear).the loads are automatically used by the structural analysis. The response and loads may be graphically assessed in animations. The analysis capabilities in Wadam comprise: Calculation of hydrostatic data and inertia properties Calculation of global responses Calculation of selected global responses of a multi-body system Automatic load transfer to a finite element model for subsequent structural analysis Master Thesis developed at West Pomeranian University of Technology, Szczecin

27 Fatigue Analysis of Offshore Drilling Unit 27 First and second order 3D potential theory for large volume structures Morison s equation and potential theory when the structure comprises of both slender and large volume parts. The forces at the slender part may optionally be calculated using the diffracted wave kinematics calculated from the presence of the large volume part of the structure. The Wadam results may be presented directly as complex transfer functions or converted to time domain results for a specified sequence of phase angles of the incident wave. For fixed structures Morison s equation may also be used with a time domain output option to calculate drag forces due to time independent current. The same analysis model may be applied to both the calculation of global responses in Wadam and the subsequent structural analysis. For shell and solid element models Wadam also provides automatic mapping of pressure loads from a panel model to a differently meshed structural finite element model. 4.4 Sestra: Sestra is the program for linear static and dynamic structural analysis within the SESAM program system. It uses a displacement based finite element method. Sestra is computing the local element matrices and load vectors, assembling them into global matrices and load vectors. The global matrices are used by algebraic numerical algorithms to do the requested static, dynamic or linearized buckling analysis. It is interfaced with other program modules of SESAM for: Finite element model generation performed by the preprocessors Load calculation performed by the hydrodynamic analysis programs Results evaluation and presentation performed by the postprocessors The analysis capabilities of Sestra are schematically illustrated in Figure 12 EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

28 P 28 Figure 12 Schematic illustration of the capabilities of Sestra (12) Structural response due to dynamic loading can be analyzed by a quasi-static method, i.e. a static analysis in Sestra with quasi-static loads. The method involves neglecting dynamic effects of the structure. A quasi-static analysis is often used when the frequency or timevariation of the load is much lower than the lowest Eigen frequency of the structure. This is also called stiffness controlled dynamics because the mass and damping forces in the structure are small compared to the forces resulting from elastic and possible inelastic strains. 4.5 Xtact: Xtract software is a FE results presentation postprocessor a high-performance general purpose model and results visualization program. Xtract presents structural analysis results in alternative ways: deformed model, contour (iso-) curves, and numeric data on model display, X-Y graphs and tabulated data. Based on stresses computed by the analysis program Xtract computes and presents derived stresses: stresses decomposed into membrane and bending parts, principal stresses and von Mises stress. In addition to its general presentation features the animation feature of Xtract is especially useful for presenting results from hydrodynamic analyses. The motion of a vessel in waves may be animated with the resulting stresses in the hull. Interactive zooming, rotating, panning and cutting allows to achieve the best view of your model. Master Thesis developed at West Pomeranian University of Technology, Szczecin

29 Fatigue Analysis of Offshore Drilling Unit Postresp: The Postresp software have been used to do statistical post-processing of general responses given as transfer functions in the frequency domain analysis performed for global response of the platform. The transfer functions have been generated by the hydrodynamic program HydroD Wadam. 4.7 Stofat: The Stofat software is a postprocessor for fatigue design. The fatigue calculations are based on responses given as stress transfer functions. The stresses are generated by hydrodynamic pressure loads acting on the model. These loads are applied for a number of wave directions and for a range of wave frequencies covering the necessary sea states. The loads are applied to a finite element model of the structure whereupon the finite element calculation produces results as stresses in the elements. Stofat uses these results to calculate fatigue damages at given points in the structural model. The program also calculates usage factors representing the amount of fatigue damage that the structure has suffered during a specific design life EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

30 P STRUCTURAL MODELLING 5.1 Modelling Set-Up The objective of the global structural modeling is to create Panel model, Morison and global structural finite element model of the drilling platform for use in hydrodynamic and subsequent structural analysis. The SI-units has been used to for modelling: Mass = [kg] Length = [m] Time = [s] Applying these units in the analysis the output will then have the following units: Force = [N] = [kgm/s 2 ] Stress = [N/m 2 ] = [Pa] St52 is used as a material which has following properties: Table 2 Material Properties of the Structural Model (St52) Material Property Value Unit Yield Stress 2.35x 10 8 Pa Density 7850 Kg/m 3 Young s Modulus 2.1x10 11 Pa Poison s Ratio 0.3 Thermal co-efficient 1.2x10-5 delc -1 Damping co-efficient 0.03 N.s/m Modelling has been done with Sesam GeniE as shown below with color-coding: Master Thesis developed at West Pomeranian University of Technology, Szczecin

31 Fatigue Analysis of Offshore Drilling Unit 31 Figure 13 Color coding of structural members Connections between deck and column, column and pontoon are modelled with thicker plate which is also presented with color coding below: Figure 14 Color coding of thickness EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

32 P 32 Global model includes following: the longitudinal stiffness of the pontoons, the axial and bending stiffness of the braces, the axial and bending stiffness of the columns, the in plane and vertical bending stiffness of the deck 5.2 Pontoon: The pontoon has been created with shell elements where upper parts are as flat plates and lower parts are as cylindrical and spherical as shown in Figure 15. Both top and bottom of the pontoon are flat and rectangular. Longitudinal bulkhead is used which runs along the longitudinal central axis of each of the pontoon. Longitudinal bulkhead and pontoon shell are key components of the pontoon. To get geometric stiffness between pontoon shell and longitudinal bulkhead couple of transvers watertight bulkheads are modelled. Combined framing is used. Local reinforcements and minor reinforcement has been omitted in the global analysis model. 5.3 Column: Figure 15 Pontoon Four Vertical circular columns are modelled to withstand global stiffness as shown below on Figure 16. Both longitudinal and transverse bulkheads are included with vertical shell plates. Master Thesis developed at West Pomeranian University of Technology, Szczecin

33 Fatigue Analysis of Offshore Drilling Unit 33 Pontoon and deck connections are possibly the critical region of stress concentration and are also modelled. 5.5 Deck: Figure 16 Column The transversal and longitudinal bulkheads are modeled along with the outer deck shell member (Figure 17). The girders and the framing system to the upper deck shell were modelled using T-section and stiffeners are modelled with L-Section bar. Local details i.e. brackets, buckling stiffeners, etc. has been neglected as they don t contribute significantly to the global strength. Figure 17 Deck EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

34 P 34 Figure 18 Deck Framing System 5.5 Derrick: Derrick that holds the drilling apparatus has been created with two different pipes (diameter) as shown on Fig. 19 that contains four vertical legs continued by the sloping legs to the top. Outer frames are created with the pipe diameter 1m and thickness 0.05m. Outer pipes are connected with crossbar pipes of 0.8 m diameter and 0.04 m thickness. El. 65 m El. 55 m El. 46 m El Figure 19 Derrick Master Thesis developed at West Pomeranian University of Technology, Szczecin

35 Fatigue Analysis of Offshore Drilling Unit Boundary Conditions: To avoid rigid body motion of a global structural model at least 6 degrees of freedom have to be fixed. Three vertical supports should be defined by springs representing the total water plan stiffness of the structure: k = ρ w. g. A w. Where Aw = is the total water plan area of the unit (m 2 ). ρ = density of water = 1025 Kg/m 3 g= gravity of 9.81m/s 2 k = 1025 kg/m 3 x 9.81 m/sec 2 x Aw = x Aw [N/m] A set of boundary conditions is illustrated in Figure, with the following restraints: 3 vertical restraints (Z) 2 transversal horizontal restraints (Y) 1 longitudinal horizontal restraint (X). In the figure the two points with fixation in Y have the same Y-coordinate and all three points have the same Z-coordinate. Figure 20 Boundary conditions (Source: DNV-RP-C103_ ) EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

36 P 36 Figure 21 Boundary conditions applied on pontoons The spring stiffness below each corner column is then kspring = k/3 = kz 1, 2, 3. In addition, horizontal supporting, in transverse and longitudinal direction, is represented by springs equal 0.1 (10% of vertical stiffness is applied in the horizontal direction) of the total vertical spring stiffness. The transverse horizontal stiffness is applied in two (2) support points, y-direction, and one (1) spring element is applied in the longitudinal, x direction. kx1 = ky1, Panel Model: A panel model is used to calculate hydrodynamic forces from potential theory. It is the part of structural model that subjected to the water. Panel model modelling a dummy hydrodynamic pressure load is applied to the wet surface. Only outer surface of pontoons and columns are taken into account as a panel model and no internal structural components and bulkheads were considered as they are not exposed to the hydrodynamic pressure. Wet surface of the structure that has defined as a panel model is shown below in Figure 22 and Figure 23 Master Thesis developed at West Pomeranian University of Technology, Szczecin

37 Fatigue Analysis of Offshore Drilling Unit 37 Figure 22 Panel Model Simple meshing techniques have been used to create the panel model. Since the model is double symmetric only one quarter of the panel model is modelled as shown in Figure 24. The remaining parts of the model are generated in Software Wadam by the yz-xz symmetry option. Figure 23 Wet Surface for Hydrodynamic Analysis EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

38 P 38 Figure 24 Meshed Panel Model 5.8 Morison Model: The Morison model consists of beam elements representing the transverse bracing. It is used to viscous damping and drag forces (Morrison forces) of the unit using Morison theory. The buoyancy and mass forces will be calculated by the panel (radiation and diffraction) model. The slender pipe sections of Morison model connects the pontoon-column assembly of the structure. The drag coefficients were assumed as a uniform numerical value of 0.7(Cd) in the horizontal and vertical axes of the semisubmersible platform for the hydrodynamic analysis. The added mass coefficient, Ca is set to 0.0 (The added mass is defined as Cm = 1+ Ca in HydroD).A default value of meshing element length was used to create the mesh model of the Morison elements as shown below in Figure 25. Figure 25 Morison Meshed Model Master Thesis developed at West Pomeranian University of Technology, Szczecin

39 Fatigue Analysis of Offshore Drilling Unit Structural Model: It consists of all the structural components including the bulkheads, the girders, bracings and other key structural connections of the drilling platform. This structural model included the panel and the Morison model along with the finite element assembly of the deck structure as illustrated in Figure 26. Figure 26 Structural Mesh Model During analysis, problems are detected on some local area elements because of inappropriate meshing (Figure 27).After checking the Sestra.lis file, bad elements shapes are identified and fine mesh is used only on the particular elements to get the appropriate result (Figure 28) Figure 27 Problematic mesh elements. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

40 P 40 Fine mesh Figure 28 Final Structural Mesh Model In the modelling of structural model a combination of dummy hydro pressure load, equipment s load and self-weight load is applied. However during the hydrodynamic analysis elements below still water lines is separated from dry elements with defined hydrodynamic pressure. Master Thesis developed at West Pomeranian University of Technology, Szczecin

41 Fatigue Analysis of Offshore Drilling Unit HYDRODYNAMIC ANALYSIS: Wave loads are computed by HydroD Wadam using Morison s equation and potential theory. Wadam is an integrated part of the SESAM suite of programs which is tailored to calculate wave loads on models created by the SESAM Genie. The results from the Wadam global response analysis stored on a Hydrodynamic Results Interface File (G-file) for statistical post processing in Postresp. The loads mapped to structural finite elements stored on the Loads Interface File (L-file) for a subsequent structural analysis in Sestra. A Hydro model was created using HydroD software to perform hydrodynamic analysis on the global structure of the drilling unit. Hydro model is shown in Figure 29 below which is consisted of finite element model of the structure. 6.1 Analysis Setup Figure 29 Hydro Model of Semisubmersible Drilling Platform Analysis was set up to get motion response and transferring the wave loads to structural model. All models including Panel, Morison and Structural model was translated by -13.5m in z directions which is considered as operating draft of the unit to ensure that mean sea level is around z= 0 as per recommendation of the software manuals. This operating draft of 13.5 m EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

42 P 42 allows an acceptable balance of net buoyancy and static forces acting on the semisubmersible model analyzed with minimal trim and heel condition. For analysis setup following steps has been defined in HydroD Wadam package: Analysis was done with frequency domain and the drag effects of the incident waves were defined as linearization by stochastic method. First direction defined with 0 degree and last direction 315 degree with respect to the platform longitudinal axis to the sea state with the step value 45 degree Table 3 Sea State Direction Set No. of Direction Direction(deg) Period is set between 0.5 to 25 sec with the step value 1 sec Spectrum is setup to Bretshneider spectrum which is also known as a 2 parameter Pierson-Moskowitz spectrum where only Hs (significant wave height) and Tp (Peak period) need to be defined. Design wave was selected based on most extreme environmental conditions of North Sea with 100 year return period. Significant Wave height Hs=13.6m and Peak Period (Tp) =16s was found for the current operation of mobile offshore units. Spreading function of exponent 2 is selected to define short crested sea where main heading is assumed to be 45 degree with respect to longitudinal axis. In short crested sea other wave directions are taken into account than the current main wave direction Hydro model is defined as floating The water depth of the location was assumed to be uniform 300 m in the central North Sea region and typical water depth for operation of drilling unit in that region. Water density, Kinematic viscosity and Gravity is also defined with the value 1025Kg/m 3, 1.19e-006m 2 /s and m/s 2 respectively. Master Thesis developed at West Pomeranian University of Technology, Szczecin

43 Fatigue Analysis of Offshore Drilling Unit 43 Sea state duration is selected as 3 hours which has been introduced as a standard time between registrations of sea states when measuring waves. In connection with stochastic response analysis, linear (Airy) theory is used Four compartments are created inside two pontoons are shown in Figure 30, which contains fluid with 900 Kg/m 3 density. Same permeability is assigned for all compartments Figure 30 Four compartments inside two pontoons To calculate wave pressure on defined point and to define grid to represent sea surface off body points are used which was then post processed in Postresp is show in Figure 31 below. The range for the off-body points was taken as a grid system between (300m, 200m) to (-300m, -200m) with an interval of 25m in x axis and 20m in y axis. Figure 31 Off body points to define sea state grid Drift forces are calculated with far field integration (horizontal) method EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

44 P 44 Higher frequency limit in HydroD is around 2, too fine mesh create too much nodes which may make size problem in Stofat so rather coarse mesh is used for global analysis Mass Model properties for the Hydrodynamic Analysis is given below in Table 4 Table 4 Mass Model Properties of the Structure Property Value Unit Mass of structural Model 23.6 x 10 6 Kg Buoyancy Volume 25.1x10 3 m 3 Centre of Buoyancy in coordinate(x,y,z) (0, 0, -13.5) m Centre of Gravity in coordinate (x,y,z) (0, 0, -10) m Radius of Gyration (x,y,z) 30.9, 29.7,38.6 m Roll-Pitch Centrifugal Moment (XYRAD) x10-14 m 2 Roll-YAW Centrifugal Moment (XZRAD) 0 m 2 Pitch-YAW Centrifugal Moment(YZRAD) x10-15 m 2 The mass model of the structure as show belong in Figure 32 was created by HydroD using user defined option of homogenous density panel model with input co-ordinate system and fill from buoyancy tab Figure 32 Mass model of the Drilling Unit Master Thesis developed at West Pomeranian University of Technology, Szczecin

45 Fatigue Analysis of Offshore Drilling Unit 45 The load-cases based on combination of wave direction and wave period analyzed for different loading conditions has been mentioned as follows Load Case Load Case Table 5 Load cases for Heading and Wave periods Heading Period Load Case Heading Period Load Case Heading Period Heading Period 2_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

46 P 46 Load Case Heading Period Load Case Heading Period Load Case Heading Period Load Case Heading Period 6_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Master Thesis developed at West Pomeranian University of Technology, Szczecin

47 Fatigue Analysis of Offshore Drilling Unit Global Motion Response Analysis: The response of the structure was measured in terms of its Response Amplitude Operators (RAOs) for the 6 degree of freedom which is presented with POSTRESP software below: Figure 33 RAO for different wave directions at relative points (0, 0, and 35) The RAO is shown above for a relative point (0, 0, and 35). There are two peaks, one at 21s (amplitude 8.1) which comes from the heave resonance mainly and one at 24s (amplitude 2.4) which comes from the roll resonance mainly. The worst wave direction is 270 and 45 degrees for both peaks. RAO for Heave, Roll and Pitch are given below in Figure 34, Figure 35 and Figure 36: EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

48 P 48 Figure 34 RAO of Heave Figure 35 RAO of Roll Master Thesis developed at West Pomeranian University of Technology, Szczecin

49 Fatigue Analysis of Offshore Drilling Unit 49 Figure 36 RAO of Pitch Since the critical wave periods are related to natural periods and viscous damping is important for the vertical motions of a semi-submersible. So Wadam was rerun after introducing some damping in heave, roll and pitch. 5% damping is given in all modes as shown in below Figure.. Figure 37 Damping Matrix The new RAO is shown in Figure 38. There are still two peaks, one at 14s (amplitude 0.6) which comes from the roll resonance and one at 24s (amplitude 1.6) which comes from the heave resonance. But now the first peak is significantly larger. EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

50 P 50 Figure 38 RAO at relative point (0, 0, and 35) after addition of damping The worst wave direction is 180 degrees for the largest peak and 270 degree for highest peak. DNV-NA scatter diagram is used and short crested sea states is assumed. From Postresp long term response is calculated for 270 degree heading as given in Table 6 Table 6 Long term response from Postresp Heading 270 Year=1 Year=5 Year=10 Year=50 Year=100 RAO Output from the HydroD Wadam was represented as a global motion response and hydrodynamic loading on the drilling unit. Surface wave loads on the platform for various loadcases based on excitation frequencies for different motions has shown below in Figure 39 to Figure 44 with contour plots. From the figures it is clear that heave, pith and roll motions are more dominant where other motions like Sway, yaw or surge motions are almost negligible. Sway or yaw motions are negligible because of symmetry of the structure and also in deep water sway motion is negligible. Master Thesis developed at West Pomeranian University of Technology, Szczecin

51 Fatigue Analysis of Offshore Drilling Unit 51 Figure 39 Surface wave loads at 180 degree heading and Hz frequency (Pitch RAO) Figure 40 Surface wave loads at 270 degree heading and Hz frequency (Heave RAO) EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

52 P 52 Figure 41Surface wave loads at 270 degree heading and Hz frequency (Roll RAO) Figure 42 Surface wave loads at 180 degree heading and Hz frequency (Surge RAO) Master Thesis developed at West Pomeranian University of Technology, Szczecin

53 Fatigue Analysis of Offshore Drilling Unit 53 Figure 43 Surface wave loads at 270 degree heading and Hz frequency (Sway RAO) Figure 44 Surface wave loads at 315 degree heading and Hz frequency (YAW RAO) EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

54 P GLOBAL STRUCTURAL STRENGTH ANALYSIS The hydrodynamic load is transferred to structural model (Figure 45) for subsequent quasistatic analysis of the structure. The rigid body motions of the model were restrained by means of applying spring elements to provide the required balance of forces to the structural loading. The load cases of the structure are listed below: Self-weight of the structure Equipment s which are positioned symmetrically in four positons of the structure, each of them are 15000Kg, 4m height, 3m length and 5m width Hydrodynamic loads from WADAM analysis Four mass points at top of the derrick where each mass point has a mass of 2.0E5 Kg The local effect of the wind and current loads on the structure is considered negligible compared to extreme wave loading of 100 year return period Figure 45 Global structural model The quasi- static structural analysis were done for different wave frequencies and headings. Some von-misses stresses are plotted below with XTRACT software tool in Figure 46 and Figure 47. The von Mises stresses are low, since the structure response from only one frequency can be considered in a frequency domain analysis but the wave load is a sum of frequencies that are in different phases relative to each other. Combined effects are considered in later section during fatigue analysis. Master Thesis developed at West Pomeranian University of Technology, Szczecin

55 Fatigue Analysis of Offshore Drilling Unit 55 Figure 46 Von-Misses Stress at 45 degree wave heading and Hz excitation frequency Figure 47 Von-Misses Stress on Column EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

56 P GLOBAL FATIGUE ANALYSIS AND RESULT: Stofat software tool is used for spectral fatigue analysis which is followed by frequency domain hydrodynamic analysis from HydeoD Wadam and quasi-static structural analysis from Sestra that was executed earlier. Harmonic waves of unit amplitude at different frequencies and directions are passed through the structure and generate a set of stress transfer functions which are read into Stofat through the Result Interface File and used in the long term stochastic fatigue calculations. In spectral method, long term fatigue calculation is based directly on a scatter diagram, response spectrum and SN-curves as input. SN curve is used to define the fatigue characteristics of a material subjected to repeated cycle of stress of constant magnitude. The wave climate is presented by a scatter diagram representing North Atlantic which provides the frequency of occurrence of a given parameter pair (e.g. (HS, Tz)) as shown in Figure 48 below: Figure 48 Scatter Diagram for the North Atlantic (Source: DNV-RP-C205) Master Thesis developed at West Pomeranian University of Technology, Szczecin

57 Fatigue Analysis of Offshore Drilling Unit 57 The SN-curve delivers the number of cycles required to produce failure for a given magnitude of stress (Figure 49). DNVC-I SN-curve is used which is default for Stofat (Figure 50) Figure 49 DNV-SN Curves (Stofat_UM) Figure 50 DNVC-I SN curve plotted from STOFAT EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

58 P 58 A Bretschneider wave spectrum is used. Priority is given to worst usage factor. Usage factor is above 0.80 and design fatigue life is set to 20 years. Cos 2 is the wave-spreading function to define short crested sea for fatigue analysis which is plotted from STOFAT below: Figure 51 Wave Spreading function for short crested Sea STOFAT obtains the principal stresses from SESTRA and calculates the accumulated partial damage.the accumulated partial damage is weighted over sea states and 8 wave directions. The wave directions that are considered are 0, 45 up to 360 degrees. The definition of the wave direction, or heading angle, is presented in Fig. 52 Master Thesis developed at West Pomeranian University of Technology, Szczecin

59 Fatigue Analysis of Offshore Drilling Unit 59 Bow 225 Head 180 Bow 135 Beam 270 Structure Beam 90 Quarter 315 Quarter 45 Following 0 Figure 52 Definition of the wave direction (heading angle) in this investigation. The locations of the most critical elements in terms of usage factors are presented in Figure 53. The usage factor is defined as the design life - life in service - divided by the calculated fatigue life. For example, if the usage factor is 1.0, it will result in failure after 20 years, or if the usage factor is 0.5, it will result in failure after 40 years. Figure 53 Maximum Usage Factor of the Structure EMSHIP Erasmus Mundus Master Course, period of study September 2013 February 2015

60 P 60 Figure 54 Fatigue Life of Global Structure Fatigue life is shown in above Figure 54 based on element of the global structure. Fatigue life for critical connections are presented below which are the average value of adjacent elements of the connections. Table 7 Fatigue life in Critical connections Connections Fatigue Life (Years) Deck to Column Above 50 Column to Pontoon Around 30 Column to Brace Above 50 Deck to Derrick Around 40 The fatigue-critical locations are presented in Table 7 for a fatigue analysis with equal probability for all wave directions. It seems reasonable that the elements that have the maximum usage factors and minimum fatigue life are located in the column to pontoon region for combined loading conditions; column to pontoon connections is also the maximum stressed region of the unit. The Appendix B present the elements with the maximum usage factors for different wave directions and the calculated fatigue life for these elements. Master Thesis developed at West Pomeranian University of Technology, Szczecin